高原城市拉萨典型VOCs排放源成分谱特征

高海拔地区低的气压和含氧量会影响燃烧和溶剂挥发过程,进而影响挥发性有机物（VOCs）的排放特征.然而,人们对海拔3 000 m以上的燃烧源或挥发源的VOCs排放特征的认识有限.拉萨市正在经历快速的经济发展和城市化进程,VOCs排放源日趋复杂.采集拉萨市机动车尾气、汽油挥发、柴油挥发、液化石油气和燃香等排放源VOCs样品进行分析,研究成分谱特征,并与平原地区的相比较.发现液化石油气VOCs排放谱以丙烷和丁烷为主,与平原地区的一致；汽油挥发中1,2,4-三甲苯占比最高,而汽油车尾气中异戊烷、正丁烷和丙烷占比高,1,2,4-三甲苯占比较低,这不同于平原地区的结果；柴油挥发比汽油及汽油车排放具有更高占比的C7以上高碳化合物；柴油车排放中含氧挥发性有机物（OVOC）占比最高,而平原地区则是丙烯和C7以上烷烃占比高；燃香排放中OVOC占比最高,特征示踪物是乙腈,且有不容忽视的异戊二烯排放.从甲苯与苯的比值上看,汽油车及柴油车排放与平原地区的结果基本一致,但两地区柴油挥发和燃香排放则不同.从苯、甲苯和乙苯比例三元图上看,柴油挥发落在工业及溶剂排放区内,有别于平原地区的结果.与其它城市一样,烯烃和芳香烃化合物也是高原城市中具有高反应活性VOCs组分,二者分别对臭氧和二次有机气溶胶的生成有重要的贡献.初步研究结果表明,高原城市地区不同VOCs源的组分特征与平原地区有所差异,需要进一步深入研究,以应对潜在的环境和健康风险.

Composition Characteristics of Typical VOCs Sources in the Highland City of Lhasa

The low air pressure and oxygen content at high altitudes affect combustion and solvent volatilization processes, which in turn affect the emission characteristics of volatile organic compounds （VOCs）. However, there is very limited knowledge about the VOCs emission characteristics of combustion sources or volatile sources over 3000 m above sea level （asl）. Lhasa City has been undergoing rapid economic development and urbanization, where VOCs emission sources are becoming increasingly complex as a result of energy consumption changing from biomass burning to fuel combustion. Samples from major VOCs emission sources in Lhasa City, including motor vehicle exhaust, gasoline volatilization, diesel volatilization, liquefied petroleum gas （LPG）, and incense burning emissions, were collected for analysis to study their compositional spectra and compare them with those in the plains. The VOCs emission profile of LPG was found to be consistent with those in previous studies in the plain area, with dominant propane and butane； gasoline evaporation had the highest proportion of 1,2,4-trimethylbenzene, whereas gasoline vehicle emissions had high proportions of isopentane, n-butane, and propane and a low proportion of 1,2,4-trimethylbenzene, unlike what has been reported in the plain area. Diesel volatilization had a higher proportion of C7 and higher carbon compounds than gasoline volatilization and gasoline vehicle emissions. Diesel vehicle emissions had the highest ratio of oxygenated VOC （OVOC）, which was different from the high compositions of propylene and alkanes above C7 in the plains. Incense burning had the highest percentage of OVOC emissions, with acetonitrile as the tracer that distinguished it from other emission sources； it also had a small amount of isoprene emissions, which should not be overlooked. As for the ratio of toluene to benzene （T/B） in gasoline and diesel vehicle exhaust, they were generally consistent with that in the plain area. However, both T/B ratios in diesel evaporation and incense burning were >1, which differed from that in the plain area. In the ternary diagram of benzene, toluene, and ethylbenzene ratios, diesel volatilization fell within the industrial and solvent emission zones, which differed from that in the plain area. Like other cities, alkenes and aromatics were also highly reactive VOCs species in highland cities, contributing significantly to ozone and secondary organic aerosol generation, respectively. Preliminary results indicated that the composition characteristics of VOCs sources in highland areas differed from those in the plains, and further in-depth studies are needed to address their potential environmental and health risks.